Application Note

VISHAY GENERAL SEMICONDUCTOR
www.vishay.com
Rectifiers
Application Note
Solar Cell Bypass Diodes
in Silicon Crystalline Photovoltaic Panels
IMPORTANT CHARACTERISTICS OF BYPASS DIODES FOR PHOTOVOLTAIC SOLAR CELLS
1. Forward Voltage Drop (VF) at Bypass
The basic function of bypass diodes in solar cells is to
protect against hot spot damage when the photovoltaic
panel is partially shaded by snow, fallen leaves, or other
obstructions, as shown in Fig. 1.
typical operation of a diode in both forward (quadrant 1) and
reverse (quadrant 3) polarity of operation. When the diode is
in forward biased (conduction mode - turned-on) a small
drop across the device occurs and does not change much
as the forward current changes (quadrant 1). Now the device
can carry a large amount of current as long as the
temperature of its junction does not exceed Tj max. The
device turns on when the panel is fully or partially shaded
thereby bypassing the panel. In regular operation the diode
is reverse biased with a small amount of leakage current
(Fig. 2 - quadrant 3). In the solar cell bypass function, the
breakdown voltage of the diode is always higher than the
total output voltage in VOC of the solar cell in the string which
is paralleled with the bypass diode.
Fig. 1 - Bypass Operation
In this bypass operation, the forward voltage drop of the
diode is similar to its internal resistance in operation, and is
relative to the power loss of the diode and system efficiency.
The equation for power loss of a diode in bypass operation
is
Pd = VF (at rated current and temperature) x IF.
This dissipated energy is a result of a change in
temperature, and the definition of this change is RJC, RJL,
or RJA.
Fig. 2 - Voltage-to-Current Curve of Diode
2. Breakdown Voltage
Another consideration in the breakdown voltage
specifications for bypass diodes is the maximum output
voltage of solar cells in the flash condition, which simulates
the voltage rush of lightning. This breakdown voltage is
different than the isolation voltage of the photovoltaic solar
panel or the solar power system, which is specified at
IEC 61215-10-3. The bypass diode can possibly be
damaged when the output voltage of the solar cell block is
greater than the breakdown voltage from power dissipation.
Bypass diodes are in reverse biased mode (Fig. 2) during
normal operation in solar cell panels, and are engaged by
the output voltage of solar cell blocks. Fig. 2 shows the
The equation for power dissipation is Pd = VBR x IBR, where
VBR = breakdown voltage, and
IBR = current at breakdown status.
The maximum condition for one hour of the rectifier is
determined by
200 °C = (Pd x RJC, RJL, or RJA) + ambient temperature.
The continuous mode is determined by
Max. TJ = (Pd x RJC, RJL, or RJA) + ambient temperature.
Revision: 18-Aug-11
Document Number: 89398
1
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APPLICATION NOTE
The maximum junction temperature of most Schottky
rectifiers being used as bypass diodes is 200 °C for one
hour, and this limitation determines their maximum
operating current capability.
Application Note
www.vishay.com
Vishay General Semiconductor
Solar Cell Bypass Diodes
in Silicon Crystalline Photovoltaic Panels
APPLICATION NOTE
3. Reverse Leakage Current
Fig. 3 - Reverse Leakage Current in Operation
Fig. 4 - Reverse Leakage Current in Open Circuit
The reverse leakage current of a diode is related to its
reverse biased voltage and junction temperature. Schottky
rectifiers are generally used in bypass diodes for
monocrystalline silicon and polycrystalline photovoltaic
solar panels. Schottky rectifiers feature low forward voltage
drop, offering higher efficiency and current density than
traditional P-N junction diodes. However, they also have
high a leakage (Fig. 3) reverse leakage current inoperation
reliability of bypass diodes in solar panel applications. In
normal solar panel operation, the bypass diode is reverse
biased and the leakage current is constantly passing
through it, as shown in Fig. 3.
The two worst cases in solar cell operation are high current
exposure in the unloaded status during installation or repair,
and the rush current exposure during lightning. In solar cell
specifications, the highest output voltage is at the open
circuit condition. The output voltage of a solar cell in the
loaded condition is lower than in the open circuit condition
and when the panel is flash tested (a technique used to
avoid defects in the field). The reverse leakage current of
bypass diodes is increased in this condition and will cause
diode failures.
Fig. 5 - Output Voltage Form of Solar Cell at Flash Test
Revision: 18-Aug-11
Document Number: 89398
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For technical questions within your region: [email protected], [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay General Semiconductor
Solar Cell Bypass Diodes
in Silicon Crystalline Photovoltaic Panels
TMBS® (TRENCH MOS BARRIER SCHOTTKY) RECTIFIERS FOR BYPASS DIODES
A new series of Vishay diodes applies new technology for
low leakage at high temperatures and low forward voltage
drop, thus improving on the disadvantages of low- and
high-barrier conventional planar Schottky rectifiers.
The new Vishay TMBS axial-type Schottky rectifiers reach a
15 A current capability in the DO-201AD (P300) package,
and a 20 A current capability in the P600 package without a
heatsink.
Axial TMBS Rectifiers for Solar Cell Bypass Diodes
Bypass diodes in solar junction boxes experience harsh
conditions due to the poor ventilation of the enclosure and
heat radiation from a limited heatsink.
Axial-type packages are commonly used in bypass diodes,
but their current capability is limited by their chip size and
leads, which act as a heat sink in diodes.
Low forward voltage drop is a critical characteristic of
bypass diodes because of the heat they generate by power
dissipation. Low leakage current at high temperatures is
also important when bypass diodes in solar panels go to
normal mode operation from shaded mode.
VSB2045 and VSB1545
Fig. 6 - Power Axial Packages
Current bypass diodes from Vishay include the SB15H45
high-barrier type Schottky rectifier with very low leakage at
high temperatures, and the 150SQ045 low-barrier type
Schottky rectifier with very low forward voltage drop.
APPLICATION NOTE
Fig. 8 - Current Derating Curves of the P600 Package
Increasing the current rating of a solar cell through
enhanced technology or by enlarging the cell size requires a
bypass diode with a higher current rating. The VSB2045 has
a large current capability of 20 A for enhanced high-power
solar panel junction boxes.
The leakage current characteristics of TMBS rectifiers are
improved over conventional planar low-barrier Schottky
rectifiers.
Fig. 7 - Axial Rectifier in Junction Box
Revision: 18-Aug-11
Document Number: 89398
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For technical questions within your region: [email protected], [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay General Semiconductor
Solar Cell Bypass Diodes
in Silicon Crystalline Photovoltaic Panels
Fig. 9 shows the reverse leakage current of devices at low
voltages under various temperature conditions. The
SB15H45 has the very low leakage current characteristics of
high-barrier Schottky rectifiers, while conventional
low-barrier devices have high leakage
current
characteristics at high temperatures. TMBS rectifiers also
have
low
leakage
current
characteristics
at
high-temperature conditions.
Fig. 9 - Leakage Current at Low Voltages
TO-220 and TO-263 Power Package TMBS Rectifiers for Solar Panels
APPLICATION NOTE
The TO-220 and TO-263 power packages have high current
capability up to 60 A and are optimized for PCB assembly.
Fig. 10 - Power Package Types and Pin Configurations
Fig. 11 - TO-220 Bypass Diodes in a Junction Box
Fig. 10 shows the pin configurations and packages of Vishay
planar and TMBS Schottky rectifiers for bypass diodes.
Fig. 11 shows a PCB assembly with a TO-220 type rectifier
in a junction box. The current capabilities of these power
packages given in their datasheets are conditional on
heatsink and PCB pad size and air ventilation.
Revision: 18-Aug-11
Document Number: 89398
4
For technical questions within your region: [email protected], [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay General Semiconductor
Solar Cell Bypass Diodes
in Silicon Crystalline Photovoltaic Panels
Fig. 12 - Current Capability of TO-263 Schottky Rectifiers
Fig. 12 shows the current capability of several kinds of
Vishay TO-263 package type Schottky rectifiers, including
the MB3045S single planar type, MBRB3045CT dual planar
type, VBT3045CBP dual TMBS, and VBT6045CBP for solar
cell bypass applications.
The actual current capability differs from this graph
depending on the PCB pad and heat sink size of the junction
box.
Fig. 13 - Current Capability of TO-263 TMBS Schottky Rectifiers
APPLICATION NOTE
Fig. 13 shows the reverse leakage currents of Vishay TMBS
Schottky rectifiers and conventional low-barrier Schottky
rectifiers under different temperature conditions.
Revision: 18-Aug-11
Document Number: 89398
5
For technical questions within your region: [email protected], [email protected], [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000